1. Field of the Invention
This invention relates to heat-sensitive recording elements particularly useful for making color hard copy, to a method of imaging employing said elements and to novel leuco dyes (the term "leuco dye" is used herein to refer to a substantially colorless compound which generates a colored material upon heating) and the dyes derived therefrom useful as the color image-forming materials.
2. Description of the Relevant Art
Leuco dyes have been suggested which become irreversibly colored by the loss of a single group. For example, Japanese Patent Kokai No. 57-46239, Laid Open Mar. 16, 1982, discloses colorless indoaniline compounds which possess an alkyl/aryl sulfonyl group that irreversibly cleaves from the molecule upon exposure to light, usually ultraviolet light, with the result that the compound is converted to its colored form. U.S. Pat. No. 3,409,457 to Karl-Heinz Menzel discloses leuco dyes which possess an acylamino group that cleaves from the molecule upon heating to yield a colored azomethine dye. The conversion of these leuco compounds into the azomethine dyes is accelerated by using alkalis such as alkali alcoholates. The acylamino and alkyl/aryl sulfonyl groups employed in the compounds of these references depart from the molecule to effect conjugation and form a dye chromophore.
U.S. Pat. No. 4,720,449 to Alan L. Borror and Ernest W. Ellis discloses colorless di- and tri-arylmethane compounds possessing a masked acylation substituent which undergoes irreversible fragmentation upon heating to liberate the acyl group for effecting an intramolecular acylation reaction whereby the compounds are rendered colored.
The copending application Ser. No. 07/548,223 of L. D. Taylor and D. P. Waller, now U.S. Pat. No. 5,243,052, describes and claims leuco dyes comprising mixed carbonate esters of quinophthalone dyes and tertiary alkanols containing not more than about 9 carbon atoms. Application of heat to the leuco dyes causes the breakdown of at least one carbonate ester grouping in the mixed ester, whereby the compounds are rendered colored. The preferred esters are the tertiary-butoxycarbonyl (hereinafter t-Boc) derivatives.
U.S. Pat. No. 4,602,263 to Alan L. Borror, Ernest W. Ellis and Donald A. McGowan discloses the stabilization of a leuco dye by employing a tertiary-alkoxycarbonyl group, for example, t-Boc as a thermally removable protecting group. This protecting group is removed by unimolecular fragmentation upon heating, which fragmentation reaction is irreversible. Copending U.S. patent application of Roger Boggs, et al (Ser. No. 07/277,014, now abandoned and replaced by copending U.S. patent applications Ser. Nos. 07/729,420 (now U.S. Pat. No. 5,192,645) and 07/729,426, both filed on Jul. 12, 1991) discloses leuco dyes which upon the application of heat become irreversibly colored by the loss of a leaving group and a thermally removable protecting group. Both the leaving group and the thermally removable protecting group are required to stabilize the colorless leuco dyes until the application of heat.
The thermally removable protecting groups employed in the above identified U.S. Pat. No. 4,602,263 and copending U.S. patent application of Roger Boggs, et al (Ser. No. 07/277,014, now abandoned and replaced by copending U.S. patent applications Ser. Nos. 07/729,420 (now U.S. Pat. No. 5,192,645) and 07/729,426, both filed on Jul. 12, 1991) and the esters in the above-identified copending application, Ser. No. 07/548,223, now U.S. Pat. No. 5,243,057 undergo fragmentation. This fragmentation involves the formation of one or more gases (i.e., compounds which exist as gases at room temperature and atmospheric pressure), e.g., when t-Boc is employed as the thermally removable protecting group or as the preferred ester it undergoes thermal fragmentation to liberate two gases, i.e., carbon dioxide and isobutylene. These gases become trapped within the imaging system in the form of bubbles. Bubbles are undesirable because they cause light scattering and consequently impair image quality by producing areas which appear dark in transmitted light.
The present invention provides leuco dyes and methods and materials for thermal imaging which have the significant advantage of reduced bubble formation.